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A membrane is a selective barrier; it allows some things to pass through but stops others. In the case of FPSs membranes such things are water, air and water vapour molecules and particles.

FPS membranes consist of three structural layers, which combine together to form a water tight, air tight, permeable membrane.

Water tight – all FPS membranes are water tight from the outside in, so the membrane can be used as a temporary cladding for the structure.

Air tight – all FPS membranes are air tight both ways, adding to the energy efficiency of the thermal envelope.

Permeable – all FPS membranes are permeable from the inside out, so any moisture that is in the insulation or structure can freely pass through the membrane to the outside of the building.

Technical parameters of underlays and membranes

Base Weight – Is the actual weight of the product, it’s measured in grams per square meter, or g/m².  The heavier the weight of the membrane, the thicker it is, and the more durable it is.

Permeability – Is the ability to allow water vapour to pass through.  It is measured as the amount of water vapour which can pass through a square meter of membrane during 24 hours [g/m²/24h].

SD Coefficient – It compares a membrane's ability to permeate water vapour with a permeability (diffusivity) of air with a specific thickness. If the SD of a given membrane is 0.02m it means that the membrane offers the same resistance to water vapour as 0.02m thick layer of air that is 2cm. The lower the value of Sd coefficient the higher the permeability.

Water Resistance – Or head of water resistance is the water-resistance of the material when loaded with water head of the indicated height and it is normally expressed in cm or mm.  Water resistance is normally expressed by means of water resistance classes W1, W2, W3 – W1 being the highest class.

Strength – Is described by means of force [N], which has to be applied during test in order to tear a 5cm wide strip of membrane. The stronger the membrane the easier it is to install.

Sun Exposure – The membrane manufactures strengthen their products during production with stabilizers offering protection against UV radiation.  The UV radiator resistance is usually expressed in days or months.  Leaving the membranes uncovered for periods longer than recommended by the manufacturer may lead to damage.

Fire Rating – Almost everything is flammable if it’s exposed to temperatures high enough.  So different countries have developed rating systems to demonstrate how flammable different products are.  Since FPS membranes are manufactured in Europe they are tested under the European system.